You can not show a total amount (price x quantity) for the item on Item page, you can show only a price. You can show a total amount for an item (price x quantity) in price list mode or in checkout page, for example:
minimart.lotix.ru/en/price-list/price-list-formula
>Now I'tve some further problems: the user should select 1 or more pieces of a book.
If you mean field 'p_seitenzahl', then the user can select values from the value range for this field (from 32 to 400). If you mean selected quantity of an item, then You can not limit selected quantity in the current version. You can set quantity selected by default (0 or 1) using parameter "Default quantity for 'add to cart' mode" on tab "General" of global component parameters. Or you can set individual default quantity for each item on item editing page on tab "Information" parameter "Default selected quantity".
As you are the owner of IPrice minimart license, please use support forum for license owners to get support:
iprice-web.ru/en/component/kunena/iprice-mini-eng
My answers on this forum for trial version can take 5-7 days. Time limit for answers on the forum for license owners is 24 hours on weekdays, and 48 hours on holydays.